Light-emitting diodes (LEDs) can be used as backlight for various applications such as notebooks (NBs), liquid crystal display monitors (LCDMs), and liquid crystal display televisions (LCDTVs). For example, multiple LED strings including hundreds of LEDs are used in a large-size LCDTV. The LED strings are powered by a driving circuit. In order to reduce the cost of the driving circuit, the LEDs can be arrayed in LED strings coupled in parallel. As such, to have less LED strings coupled in parallel, each LED string has more LEDs coupled in series. Therefore, a voltage across the LED string can be relatively high. The relatively high voltage produces a big stress on some circuitry components in the driving circuit and the cost of the driving circuit increases due to the high voltage rating.
FIG. 1 illustrates a conventional driving circuit 100. In the example of FIG. 1, a light source driven by the driving circuit 100 includes an LED string 110 having multiple LEDs coupled in series. A boost converter 120 includes a capacitor 121, a diode 122, a switch 123, e.g., a power metal oxide semiconductor field effect transistor (MOSFET), and an inductor 124. The boost converter 120 converts an input voltage VIN, e.g., 24 volts, to an output voltage VOB, e.g., 400 volts, across the capacitor 121. A buck converter 130 is coupled between the boost converter 120 and the LED string 110. The buck converter 130 includes a capacitor 131, a diode 132, a switch 133, e.g., a power MOSFET, and an inductor 134. The buck converter 130 converts the output voltage VOB, e.g., 400 volts, of the boost converter 120 to a desired voltage VLED, e.g., 200 volts, across the LED string 110 to power the LED string 110. Therefore, the voltage across the capacitor 121, the voltage across the diode 122 and the power MOSFET 123, and the voltage across the diode 132 and the power MOSFET 133 are relatively high. Consequently, the high voltage produces a big stress on the diodes 122 and 132 and the power MOSFETs 123 and 133. Thus, the cost of the power MOSFETs 123 and 133 is relatively high due to the high voltage rating. Additionally, the boost-buck topology increases the complexity of the driving circuit 100.